I don't dispute the empirical issue (though differences between American and British law here shouldn't be overlooked),

Fair enough.

but I don't think I'm convinced on the normative front, though your examples have made me less certain of my stance.

That's all I can realistically hope for on a wide-ranging normative issue like this.

Your one-place operationalization of offence sounds reasonable, as does your unpacking of what you mean by "I don't think X should be considered offensive". (Although even with your definition, I still think there exists X such that X is both true & offensive.)